Martial arts experts use brain as well as brawn to deliver knockout punches, a study has found.
How top practitioners of karate and kung fu manage to punch so hard has long been a mystery.
Previous studies have shown that the force is not determined by muscular strength alone.
Now scientists have discovered differences in the brain structure of karate experts that are associated with punching ability.
Researchers tested the punching power of 12 black belt karate practitioners and the same number of people who exercised regularly but had no martial arts training.
To compare the two fairly, punches were measured from a short distance of only 5cm.
As expected, the karate experts punched harder. Timing appeared to be a major factor - the force they generated correlated with how well wrist and shoulder movements were synchronised.
Scans of the black belts' brains revealed microscopic differences in the cerebellum and primary motor cortex, two areas known to be involved in movement.
Study leader Dr Ed Roberts, from Imperial College London, said: "The karate black belts were able to repeatedly co-ordinate their punching action with a level of co-ordination that novices can't produce.
"We think that ability might be related to fine-tuning of neural connections in the cerebellum, allowing them to synchronise their arm and trunk movements very accurately."
The structural changes seen in the brain affected "white matter" consisting of bundles of nerve fibres.
They correlated with the age at which the experts began training and their total experience. On average, they had been training for 13.8 years.
The study, part funded by the Medical Research Council and Wellcome Trust, appears in the journal Cerebral Cortex.
"We're only just beginning to understand the relationship between brain structure and behaviour, but our findings are consistent with earlier research showing that the cerebellum plays a critical roles in our ability to produce complex, co-ordinated movements," Dr Roberts added.